Coupling between TMS pulses and all receive-only RF coils

[BenInglis]

We have determined that there is strong coupling between single pulse TMS (with a biphasic pulse of 250 usec duration) and a receive-only RF coil when the RF coil is receiving signal, i.e. there is a pronounced coupling visible when the TMS pulse is coincident with the k-space readout echo train in EPI.

[BenInglis]

Tests on 10/19/2015

SETUP: Birdcage coil, Siemens cylindrical phantom on a thin foam holder, TMS coil suspended from the upper rungs of the birdcage coil several cm above the phantom. Biphasic TMS pulse of duration 250 usec, set at 70% amplitude, one TMS pulse per TR incremented 0.1 ms each time. Timing of TMS pulses and EPI sequence as listed in Issue #5. The EPI acquisition was initiated and then the phantom was pushed out of the birdcage coil so that only noise images would result for the remainder of the time series. In the total image time series it is possible to see the displacement of the phantom as well as a drop in total noise after the magnet room door is closed. In some runs one or more images of the mosaic show structure at the moment the door is closed.

DATA FILE: tmsMRI_sequence_70pcTMS_phantom_removed, scan 3.

RESULTS: A complete set of 490 TR periods was acquired even though TMS pulses end at about volume number 370. This was to enable a long period of noise without TMS pulses. We observed intense coupling of the TMS field to the receive coil when the TMS pulse was coincident with the k-space readout period. An example frame:

Interestingly, we also observed a slow increase in what looked like white noise in the target slice when the TMS pulse occurred before the start of k-space readout. The intensity of the noise in the target slice increased linearly with decreasing time from the TMS pulse to k-space. Other slices showed a slower linear increase in noise over the same time window:

We aren't sure whether, or how, this noise variation might be related to the coupling of the TMS field to the receive coil during reception, but one suggestion is that there is a capacitance being created and charged up by the TMS pulse.

CONCLUSION: There is a strong coupling between the TMS pulse field and the receive mode of the birdcage coil.

[BenInglis]

Tests on 10/19/2015

SETUP: 12ch coil, Siemens cylindrical phantom on a thin foam holder, TMS suspended from the upper rungs of the 12ch coil a few cm above the phantom. Biphasic TMS pulse of duration 250 usec, set at 70% amplitude, one TMS pulse per TR incremented 0.1 ms each time. Timing of TMS pulses and EPI sequence as listed in Issue #5. The EPI acquisition was initiated and then the phantom was pulled out of the birdcage coil so that only noise images would result for the remainder of the time series. In the total image time series it is possible to see the displacement of the phantom as well as a drop in total noise after the magnet room door is closed. In some runs one or more images of the mosaic show structure at the instant the door is closed.

DATA FILE: tmsMRI_sequence_70pcTMS_phantom_removed, scan 3.

RESULTS: For TMS pulses coincident with k-space we obtained the same general results as for the birdcage coil in the previous comment, i.e. an intense coupling of the TMS field to the receive field. However, we didn't observe the trending RF noise in the period before initiation of signal reception.

CONCLUSION: The TMS coil couples to the 12ch coil strongly during signal reception.

[danshel]

So one of the questions now is whether the currents that the TMS pulse induces in the birdcage coil are large enough to do any damage to the components of the birdcage.

A paper by Navarro De Lara (Mag Res Med 2014, "A Novel Coil Array for Combined TMS/fMRI Experiments at 3 T") has this to say about these currents and the voltages across components:

"The theoretical voltage, as calculated from Faraday’s law, induced in a single loop by a 1 T amplitude TMS pulse at 3 kHz, was 53 V. Measurements using the simple wire loop and the TMS coil at 100% of its current rating resulted in a voltage of 33 V. Using this value in circuit simulation resulted in a coil current of 9 mA and a voltage of 0.4 V at the preamplifier input (Fig. 6c). Measurements showed a voltage of 0.7 V at the preamplifier input, dropping further to 50 mV after the preamplifier DC block input capacitor. "

The 53V value of the EMF agrees with my back-of-the-envelope calculation but are the voltages and currents really within safe (for the receive coil) operating limits for the components of the coil and preamps?

[BenInglis]

Siemens says that the bias voltage at the preamp is 10 V. Provided the voltage generated by the TMS doesn't exceed +/-10 V at the preamp then it should be okay.

[danshel]

Even if it varies temporally at around 4 KHz?

On Fri, Oct 30, 2015 at 9:34 AM, BenInglis notifications@github.com wrote:

Siemens says that the bias voltage at the preamp is 10 V. Provided the voltage generated by the TMS doesn't exceed +/-10 V at the preamp then it should be okay.

— Reply to this email directly or view it on GitHub https://github.com/Brain-Imaging-Center/TMS-fMRI/issues/10#issuecomment-152580056 .